Spatial steering with quadruple electrodes in 27 MHz capacitively coupled interstitial hyperthermia

Purpose: The 27 MHz Multi Electrode Current Source (MECS) interstitial hype rthermia system uses probes consisting of multiple independent electrodes, 10-20 mm long, to steer the 3-D power deposition. Seven point thermocouples integrated into the probes provide matching 3-D temperature feedback data. To improve spatial steering the number of independent segments was increas ed; the feasibility and reliability of four independent electrodes integrat ed into a single probe were evaluated, with special attention to efficiency and to interference between separate electrodes. Methods: The contribution of secondary coupling on the apparent electrode i mpedance and the dependence of cross coupling on the distance between leads , thermocouple and electrodes are computed using simple analytical models. The effect of this secondary coupling was assessed experimentally by compar ing power delivery by dual and quadruple electrodes, and by quadruple elect rodes in different electrode configurations (segment length 10 or 20 mm) in a nylon catheter in a muscle equivalent medium. Results: Cross coupling with the thermocouple and other electrodes was comp uted to be of the same magnitude as the primary coupling for a quadruple el ectrode. Fortunately, this does not affect operation of the electrode, ther e was no difference in performance between quadruple and dual electrodes, a nd the output power was effectively independent of the electrode configurat ion. Conclusion: Quadruple MECS electrodes for improved 3-D power control are fe asible.